The aim of this paper is to bring to light Aristotle’s knowledge of marine invertebrate diversity as this has been recorded in his works 25 centuries ago, and set it against current knowledge. The analysis of information derived from a thorough study of his zoological writings revealed 866 records related to animals currently classified as marine invertebrates. These records corresponded to 94 different animal names or descriptive phrases which were assigned to 85 current marine invertebrate taxa, mostly (58%) at the species level. A detailed, annotated catalogue of all marine anhaima (a = without, haima = blood) appearing in Aristotle’s zoological works was constructed and several older confusions were clarified. Some of Aristotle’s “genera” were found to be directly correlated to current invertebrate higher taxa. Almost the total of the marine anhaima were benthic invertebrates. The great philosopher had a remarkable, well-balanced scientific knowledge of the diversity of the various invertebrate groups, very similar to that acquired by modern marine biologists in the same area of study. The results of the present study should be considered as a necessary starting point for a further analysis of Aristotle’s priceless contribution to the marine environment and its organisms.